Light-valves are implemented in a wide variety of display technologies. For example, display and microdisplay panels are gaining in popularity in many applications such as televisions, computer monitors, point of sale displays, personal digital assistants and electronic cinema to mention a few applications.
Many light valves are based on liquid crystal (LC) technologies. In some LC technologies light is transmitted through an LC device (panel), while in others the light traverses the panel twice, after being reflected at a far surface of the panel.
In operation, an external field or voltage is used to selectively rotate the axes of the molecules of the LC material. As is well known, by application of this voltage across the LC panel, the direction of the LC molecules can be controlled and the state of polarization of the reflected light is selectively changed. As such, by selective switching of the transistors in the array, the LC medium can be used to modulate the light with image information.
In many LCD systems, light modulation results in dark-state light at certain picture elements (pixels) and bright-state light at others, where the polarization state governs the amount of light transmitted by each pixel element. Thereby, an image is created on a screen by the selective polarization transformation by the LC panel and optics to form the image or ‘picture.’
While LCD devices are becoming ubiquitous in display and microdisplay applications, there are certain drawbacks associated with known devices. For example, in known devices some of the light from the light source may be irrecoverably lost and the overall brightness of the image adversely impacted.
This loss of light in display systems may be from a variety of sources within the display system. In many LCD systems, the light from a source is selectively polarized in a particular orientation prior to being incident on the LC panel. This selective polarization may be effected by an absorptive polarizer. The LC panel may have a voltage selectively applied to orient the molecules of the material in a certain manner. The polarization of the light that is incident on the LC panel is selectively altered upon traversing through the LC layer. Light in one linear polarization state is transmitted by a polarizer (often referred to as an analyzer) as the bright state light; while light of an orthogonal polarization state is absorbed by the analyzer as the dark-state light. As can be appreciated, approximately one-half of the light energy is lost in such a system, due to absorption by the first linear polarizer.
In other display and microdisplay structures, a reflective polarizer may be used for polarization selectivity. Sometimes, the reflective polarizer is placed between a light source and the LC panel in order to provide light (polarization) recycling, wherein one polarization state is desirably transmitted by the reflective polarizer and one polarization state is reflected back toward the light source. In known systems using reflective polarizers, there are competing objectives that impede the polarization recycling. These competing objectives include light transmission efficiency and polarization extinction. For example, wire-grid polarizers may be used as the reflective polarizer. Known wire-grid polarizers have a relatively high extinction ratio. Such known wire-grid polarizers are not desirable in light recycling because the total light throughput is unacceptably low. This can result in unacceptable contrast and brightness in the viewed image.
Alternatively, plastic reflective polarizers comprising multilayer films or immiscible layered materials may be used in displays. Known plastic polarizers have a relatively low extinction ration and relatively high transmission. Such known plastic polarizers are not desirable in light recycling when used with very strong backlights as the plastics can ‘yellow’ due to light absorption. This can result in unacceptable coloration of the viewed image. As such, the need for improved light throughput and stability of coloration has not been met via known devices and structures. What is needed therefore is an apparatus that overcomes at least the drawbacks of known devices described above.